The membrane-bound enzymes, which constitute the complex but integrated electron transport system in Azotobacter vinelandii are being characterized enymatically. It has bee possble to establish that in addition to NADH and succinate oxidation, the electrn transport system of Azotobacter carries out other bioenergetically important oxidations, that are (a) cyanide sensitive, and (b) non-pyridine nucleotide dependent which involve the L-malate, L-glutamate; and it has been possible t isolate and purify the highly active cytochrome (terminal) oxidase. The terminal oxidase is tightly bound to the electron transport particle and consists of a cytochrome c4 plus c5:cytochrome o complex. The highly purified oxidase functtons maximally without phospholipid addition, although 20-30 percent activation does occur by addition of non-ionic detergents to theassay system. In contrast to this the L-malate oxidoreductase, has also been partially purified and consists of a flavoprotein-cytochrome complex and the solubilized enzyme requires phospholipid for activity. The phospholipid acitivation respone is absolute but non-specific and a variety of phospholipids do activate maximally the L-malate oxidoreductase. Attempts are also being made to find mutants of Azotobacter that have low terminal and respiratory rate capabilities, in the hope that such mutant cells will have membrane alterations that can be examined ultrastrcturally as well as enzymatically. The mutants that are of primary interest are those that are respiratory-deficient and not capable of carrying out nitrogen-fixation.